1. Field of the Invention
This invention relates to a power converter, and more particularly to a power converter in which a plurality of neutral point clamped (NPC: Neutral Point Clamped) inverters are connected in common to a DC voltage source and which supplies the AC power obtained by these inverters to a load.
2. Description of the Related Art
In prior art, there are some systems composed of two NPC inverters which have a common DC power source, as shown in FIG. 11 as an example. A rectifier 2 is connected to a 3-phase AC power source 1, and a DC smoothing capacitor 15 composed of a positive side smoothing capacitor 3 and a negative side smoothing capacitor 4. Also, a first NPC inverter 5 and a second NPC inverter 6 are connected in parallel to DC smoothing capacitor 15.
The output sides of NPC inverters 5 and 6 are connected to a motor 91, which has two sets of 3-phase windings with the same number of polarities, as shown in FIG. 10. The AC power obtained by NPC inverters 5 and 6 is supplied to motor 91. Specifically, the U, V and W phase outputs of first NPC inverter 5 are connected to windings U1, V1 and W1 of motor 91, respectively. Also, the U, V and W phase outputs of second NPC inverter 6 are connected to windings U2, V2 and W2 of motor 91, respectively.
Gate signals are supplied to gates of NPC inverters 5 and 6 from a gate signal generating system composed of a controller 10, first and second comparators 12 and 13 and a carrier generation circuit 14. A motor speed of motor 91 detected by a speed detector 8 is fed back to controller 10 which composes a 3-phase voltage reference generation system 92. Speed control is exercised therein using this feedback data and an inputted speed reference. Motor currents of motor 91 detected by current detectors 9 connected to the output side of NPC inverter 5 are also fed back to controller 10 and current control is exercised therein. A 3-phase voltage reference Vref1 (only one phase component thereof is shown), which is the current control output, is outputted from controller 10. This 3-phase voltage reference Vref1 is inputted to each of comparators 12 and 13, and is compared in amplitude with a carrier generated by carrier generation circuit 14 therein. Thus, the gate signal for NPC inverter 5 and the gate signal for NPC inverter 6 are generated in comparators 12 and 13, respectively. Details of the system shown in FIG. 11 are described in U.S. Pat. No. 5,274,542 issued on Dec. 28, 1993.
As shown in FIG. 11, NPC inverters 5 and 6 are connected to rectifier 2. Therefore, compared with a composition in which one NPC inverter is connected to rectifier 2, the current capacity of this system is doubled. But, there is a problem in this system that the potential fluctuation of the neutral point, which is the mid-point of DC smoothing capacitor 15, also becomes doubled.